According to the related art, quantum dots are fabricated through a dry chemical scheme, in which the quantum dots are fabricated by using a metal organic chemical vapor deposition (MOCVD) based on the lattice mismatch with respect to a substrate prepared under the vacuum state. The dry chemical scheme has the advantage in that nano particles formed on the substrate can be simultaneously arranged and observed. However, the dry chemical scheme requires expensive synthesis equipment and makes it difficult to synthesize the quantum dots having the uniform size in large quantity. In order to solve the above problem, a wet chemical scheme has been developed, in which the quantum dots having the uniform size are synthesized by using a surfactant.
According to the wet chemical scheme for fabricating the quantum dots, the nano particles are prevented from being conglomerated by the surfactant and the adsorption degree between the crystal surface of the nano particle and the surfactant is adjusted to synthesize the quantum dots having the uniform size and various shapes. In 1993, Bawendi Group has succeeded, for the first time in the world, the synthesis of CdSe quantum dots having the uniform size through the wet chemical scheme by using tri-octylphosphineoxide (TOPO) and trioctylphosphine (TOP) as surfactants and dimethylcadmium ((Me)2Cd) and selenium as semiconductor precursors. In addition, Alivisators Group has developed a method of synthesizing CdSe quantum dots in a more safety manner by using hexadecylamine (HDA), trioctylphosphineoxide, and tri-octylphosphine as surfactants and cadmium oxide (CdO) and selenium as semi-conductor precursors.
Thereafter, various studies and research have been pursued to form a shell on a surface of CdSe by using a semiconductor compound having a higher bandgap in order to improve the light emission characteristic of the quantum dots and to enhance the photo stability and environmental stability. For instance, the semiconductor compound includes CdSe/ZnS, CdSe/ZnSe, CdSe/CdS, and ZnSe/ZnS (see, Korean Patent Registration No. 10-0376405).
However, in the case of the quantum dot having the core/shell structure, if the shell is thick, an interfacial surface may become unstable due to the lattice mismatch between a core semiconductor material and a shell semiconductor material, so the quantum efficiency may be lowered. For this reason, the shell is fabricated in a thin thickness. Therefore, although the shell material can stabilize the surface state of the core quantum dot, it may not transfer electrons and holes to the core after absorbing light, so there are limitations in terms of the light efficiency and photo stability of the quantum dot and the environmental stability.